U.S. Pat. No. 2,379,443 discloses a variety of methods for rendering paper which is identifiable for protection against counterfeiting. The patent describes the use of colored fibres and fluorescent fibres for distinguishing between the identifiable paper and ordinary paper. The patent discloses a method of incorporating chemically-treated paper which change color upon subsequent treatment.
U.S. Pat. No. 3,880,706 discloses a method of manufacturing identifiable paper by incorporating a thermoplastic material by fusing the thermoplastic to the paper fibres during the paper-making process. Plastic dots of various colors can be incorporated within the paper fibres by the method of this patent.
Also known is the use of fine plastic security thread within the paper structure. However, when security elements of exteemely fine detail, such as a diffraction grating structure, are to be employed high temperatures must be avoided in order not to distort the fine detail of the security element.
The purpose of this invention is to provide methods and apparatus for incorporating security devices having fine detail or distinctively treated fibres within a paper web during the paper-making process without the requirement of heating the device to cause it to adhere to the paper.